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Lambda in REPL (using object-wrappers) + concurrency = deadlock #9076

scabug opened this issue Jan 11, 2015 · 12 comments

Lambda in REPL (using object-wrappers) + concurrency = deadlock #9076

scabug opened this issue Jan 11, 2015 · 12 comments


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@scabug scabug commented Jan 11, 2015

The following code is a test of generating and using java.util.Spliterator to create a

package xp

import java.util.Spliterator
import java.util.function.Consumer

class Aspi(a: Array[String], private val i0: Int, private var i1: Int) extends Spliterator[String] {
  private var i = i0
  def characteristics = Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.IMMUTABLE
  def estimateSize = i1-i
  override def getExactSizeIfKnown = i1-i
  def tryAdvance(f: Consumer[_ >: String]): Boolean = {
    println(s"Trying at $i [$i0, $i1)")
    if (i >= i1) false
    else { f.accept(a(i)); i += 1; true }
  def trySplit(): Aspi = {
    if (i1-i < 4) {
      println(s"Could not split [$i $i1)")
    else {
      println(s"Splitting from [$i $i1)")
      val child = new Aspi(a, (i+i1+1)/2, i1)
      i1 = child.i0

object Test { def main(args: Array[String]) {
  def nu = new xp.Aspi(Array("one", "two", "three", "four", "five"), 0, 5)
  val st =, true)
  val c = new java.util.Comparator[String]{ def compare(s: String, t: String) = s compareTo t }
  println([String]((s: String) => s + s).max(c))

This compiles and runs successfully. However, when the contents of main are entered in the REPL, the last line hangs. Stack trace:

"ForkJoinPool.commonPool-worker-1" #14 daemon prio=5 os_prio=0 tid=0x00007f966855f800 nid=0x721f in Object.wait() [0x00007f964a18b000]
   java.lang.Thread.State: RUNNABLE
	at $line6.$read$$iw$$iw$$anonfun$1.apply(<console>:11)
	at $line6.$read$$iw$$iw$$anonfun$1.apply(<console>:11)
	at xp.Aspi.tryAdvance(Test.scala:14)
	at java.util.Spliterator.forEachRemaining(

Explicitly creating the SAM like so:

println([String](new java.util.function.Function[String, String] {
  def apply(s: String) = s + s

solves the issue. (Note: I have not tried this on 2.11.5; I assumed 2.12.x was up to date in anything relevant.)

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@scabug scabug commented Jan 11, 2015

Imported From:
Reporter: @Ichoran
Affected Versions: 2.12.0

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@scabug scabug commented Jan 13, 2015

@retronym said:
Enabling -Xprint:all in the REPL, shows the difference between your manually expanded version and the SAM-lambda version:

  object iw extends Object {
    private[this] val res0: scala.runtime.BoxedUnit = _;
    <stable> <accessor> def res0(): Unit = ();
    final def apply$body$1(s: String): String = s.+(s);
    def <init>(): type = {
      iw.this.res0 = {
          new <$anon: java.util.function.Function>()
  @SerialVersionUID(value = 0) final class anonfun$1 extends Object with java.util.function.Function with Serializable {
    final override <synthetic> def apply(s: String): String = $line3.iw.apply$body$1(s);
    final override <synthetic> <bridge> <artifact> def apply(x$1: Object): Object = anonfun$1.this.apply(x$1.$asInstanceOf[String]());
    def <init>(): <$anon: java.util.function.Function> = {

The apply method delegates to iw.apply$body$1. This blocks when called form the threadpool because the static initializer of iw$ has not completed, it is blocking on the entire operation to complete.


Indeed this terminates again under scala -Yrepl-class-based. But we can't switch to that as a default until we sort out problems like:

% qscala -Yrepl-class-based

scala> class C(val a: Any) extends AnyVal
<console>:7: error: value class may not be a member of another class
       class C(val a: Any) extends AnyVal
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@scabug scabug commented Jan 13, 2015

@retronym said:
BTW, a workaround is to indirect through a lambda:

scala> () => Test.test(s => s + s)
res0: () => Unit = <function0>

scala> res0()
Splitting from [0 5)
Could not split [0 3)
Could not split [3 5)
Trying at 3 [3, 5)
Trying at 0 [0, 3)
Trying at 1 [0, 3)
Trying at 4 [3, 5)
Trying at 2 [0, 3)
Trying at 5 [3, 5)
Trying at 3 [0, 3)
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@scabug scabug commented Jan 13, 2015

@retronym said:
See also:!topic/scala-user/fljtxY9bJV0

/cc @som-snytt Do you think we could encode my workaround above into the object based wrappers to make the default REPL less prone to this problem?

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@scabug scabug commented Jan 13, 2015

@som-snytt said:
How about making it an App?


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@scabug scabug commented Aug 19, 2016

@SethTisue said:
@retronym should this be retargeted for 2.12.1 or Backlog?

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@scabug scabug commented Mar 1, 2017

Dave Gurnell (davegurnell) said (edited on Mar 1, 2017 10:35:25 AM UTC):
Seth suggested I post this recreation here. Try this on a 2.12.1 REPL:

import scala.concurrent._
import scala.concurrent.duration._

// This works fine:
val x = Future(42)
Await.result(x, 1.second)

// This throws a TimeoutException:
Await.result(Future(42), 1.second)

Adding scalacOptions += "-Ydelambdafy:inline" to build.sbt makes the problem go away.

The problem can also result in a NoClassDefFoundError as shown here (running Tut on an ebook project):

[tut] compiling: /Users/dave/dev/projects/advanced-scala/src/pages/
java.lang.NoClassDefFoundError: Could not initialize class $line4.$read$$iw$$iw$$iw$$iw$$iw$$iw$
    at scala.runtime.java8.JFunction0$mcI$sp.apply(JFunction0$mcI$
    at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:653)
    at scala.util.Success.$anonfun$map$1(Try.scala:251)
    at scala.concurrent.Future.$anonfun$map$1(Future.scala:287)
    ... manually elided for brevity
[tut] *** Error reported at /Users/dave/dev/projects/advanced-scala/src/pages/monads/
java.util.concurrent.TimeoutException: Futures timed out after [1 second]
  at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:255)
  at scala.concurrent.impl.Promise$DefaultPromise.result(Promise.scala:259)
  at scala.concurrent.Await$.$anonfun$result$1(package.scala:190)
  at scala.concurrent.BlockContext$DefaultBlockContext$.blockOn(BlockContext.scala:53)
  at scala.concurrent.Await$.result(package.scala:123)
  ... 88 elided
[error] (run-main-0) java.lang.Exception: Tut execution failed.
[trace] Stack trace suppressed: run last *:streams for the full output.
[trace] Stack trace suppressed: run last *:tutOnly for the full output.
[error] (*:tutOnly) Nonzero exit code: 1
[error] Total time: 5 s, completed 27-Feb-2017 09:38:19
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@adriaanm adriaanm commented Jun 26, 2017

Probably a wontfix in 2.12, but should consider this as we improve the repl in 2.13 (where we'll standardize on some form of class-based wrapping).

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@SethTisue SethTisue commented Sep 11, 2018

should this still be on the 2.13.0-RC1 milestone, or shall we push it out to 2.13.x or 2.14?

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@smarter smarter commented Sep 12, 2018

It's indeed fixed in Dotty: scala/scala-dev#195 :)

@adriaanm adriaanm modified the milestones: 2.13.0-RC1, 2.14.0-M1 Jan 8, 2019
@adriaanm adriaanm modified the milestones: 2.14.0-M1, 2.13.2 Nov 26, 2019
@adriaanm adriaanm modified the milestones: 2.13.2, 2.13.3 Nov 26, 2019
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@adriaanm adriaanm commented Nov 26, 2019

Re-scheduling for 2.13 in the context of scala/scala-dev#325 and a commercial support request.

First we should finish the jline 3 upgrade, but then let's look into improving the encoding.

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@adriaanm adriaanm commented Dec 3, 2019

@retronym suggested a few alternative solutions -- one is to type check using the current object wrappers, and then have a repl phase that does the class-based encoding? This would avoid breaking anyone that relies on the current repl tree shapes during typer. Have to check how this affects serialization in Spark.

@SethTisue SethTisue changed the title Lambda in REPL (using object-wrappers) gives non-termination of Java 8 Stream Lambda in REPL (using object-wrappers) + concurrency = deadlock Dec 24, 2019
@dwijnand dwijnand assigned dwijnand and unassigned adriaanm Feb 12, 2020
@dwijnand dwijnand added the has PR label Feb 24, 2020
@dwijnand dwijnand modified the milestones: 2.13.3, 2.13.2 Feb 24, 2020
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